Extrusion blow molding is useful for making hollow parts or structures having a variety of uses. Containers such as tanks, pressure vessels, cylinders and bottles, which may be used for pressurized or compressed gases and cryogenic gases, may be conveniently formed by extrusion blow molding.
Liquid crystal or crystalline polymers (LCPs) are beneficially characterized by gas barrier properties, chemical resistance or inertness, high creep resistance, excellent retention of properties over a broad temperature range including lack of embrittlement at cryogenic temperatures, and a low coefficient of thermal expansion. However, liquid crystalline polymers are difficult to process by extrusion blow molding.
According to Plastics World, p. 11, July 1988 and Adv. Mater., 10 (10), p. 2, Jun. 27, 1988, a moldable LCP is known. The LCP may be crystalline and moldable in 30% and 50% glass-reinforced formulations, or may be amorphous and extrudable, blow moldable and injection moldable in neat, glass-reinforced and glass/mineral formulations.
In continuous extrusion blow molding, parisons are continually formed and molds move into place to accept tubes of molten polymer. Continuous extrusion blow molding to impart biaxial orientation to a part made from a copolyester LCP (hydroxybenzoic acid and 6-hydroxy-2-naphthoic acid), has been attempted. In this regard, Blizard and Baird, Intern. Polymer Processing, 4:172-178 (1989), report forming a small part having no observed biaxial orientation, and rapid solidification to be a processing limitation. However, they report success and the benefit of relatively slow crystallization for a copolyester made from hydroxybenzoic acid and polyethylene terephthalate.
Processing difficulties are accentuated when a large-sized part is desired, and also when uniformity of wall thickness is important. Therefore, there is a need for an improved process for forming a liquid crystalline polymer into a hollow part or structure by extrusion blow molding.